A ceramic filter made up of a ceramic porous body and a separation film filter with an inorganic porous film (alumina, silica, zeolite, etc.) formed on a surface of the ceramic porous body are used for separation and filtration of gas and liquid in petrochemical, food chemistry, and energy industries. The ceramic porous body as described above is desired to have sufficient mechanical strength as well as a largest possible pore diameter and highest possible porosity so as to allow passage of large amounts of gas and liquid and therefore must be fired by using ceramic raw material having a comparatively large particle diameter; however, if a particle diameter is large, a material with high refractoriness like, for example, alumina particles must be sintered at a comparatively high temperature, for example, equal to or greater than 1700 degrees C. for acquiring necessary strength. Thus, to take measures against the problem as described above, for example, Patent Document 1 proposes a method of adding a certain amount of an organic pore forming agent and Patent Documents 2, 3, etc. propose methods of using an appropriate amount of a sintering auxiliary agent (such as a glass component, SiO2, MgO, and ZrO2). Patent Document 4 proposes a method of mixing coarse-particle alumina (5 to 50 μm) and fine alumina (2 μm or less) and firing the mixture at 1600 degrees C. lower than 1700 degrees C. so as to maintain high strength while high porosity is maintained.
Conventionally, to manufacture a two-layer structure ceramic filter with an inorganic porous film of fine-particle alumina etc. formed on the ceramic porous body as described above, slurry containing particles having a diameter smaller than particles making up the ceramic porous body is adjusted and applied onto a surface of the once-fired ceramic porous body, and the ceramic porous body is then fired again to sinter the slurry and acquire a porous thin film. However, as lower costs, higher performance of film material, and thinner films are realized, it is desired that the manufacturing of the two-layer structure ceramic filter as described above includes, for example, a step of applying the slurry onto a green molded body of a ceramic porous body molded into a predetermined shape from a material acquired by kneading ceramic particles with a molding auxiliary agent added thereto and simultaneously firing the green molded body and the slurry. The green molded body is an unfired ceramic porous body molded into a predetermined shape from a material acquired by kneading ceramic particles with a molding auxiliary agent added thereto, for example.
However, when the slurry is applied onto the green molded body as described above and the green molded body and the slurry are simultaneously fired, since a firing temperature for sintering slurry typically made of fine-particle alumina into a porous thin film is equal to or less than 1450 degrees C. and is comparatively low, if the firing is performed at, for example, a firing temperature of about 1600 degrees C. for firing the alumina porous body as described in Patent Document 4, the porous thin film is excessively sintering, resulting in progression of densification, and can no longer be applied to a ceramic filter, which is a problem.